A molecule is prochiral if the addition of a new group or an exchange of one group on the molecule would create a new stereocenter and, therefore, a chiral molecule. A prochiral atom must be bonded to three different groups before any change is made.
For example, consider the following molecules.
The molecule on the left is prochiral
because a new stereocenter can be made by replacing
one group on the carbon marked with an asterisk (*) with a new one. The molecule on the right is prochiral
because a new stereocenter can be made by adding
a new group to the carbon marked with an asterisk.
to see these changes and learn more about them.